PROJECT SUMMARY/ABSTRACT The long term goal of our research is to better understand the molecular mechanisms regulating hepatocyte proliferation during liver regeneration. The liver possesses an innate capability to repair itself after damage, restoring its original structures and functions. Timely or enhanced liver repair or regeneration leads to recovery and survival, whereas delayed or inhibited liver repair or regeneration results in unrestrained injury and death. Remarkably, hepatocyte proliferation constitutes the first line of repair response to acute and chronic liver injuries and is the driving force for liver regeneration. Therefore, understanding how hepatocyte replication is regulated is of paramount importance in developing clinical strategies to treat liver injuries. Various growth factors and cytokines form complex regulatory networks and coordinately modulate the proliferative response of hepatocytes to massive liver mass loss. However, the precise molecular mechanisms that control the initiation, progression, and termination of the hepatocyte cell cycle during the course of liver repair remain elusive. Hence, the objective of this proposal is to further understand the molecular mechanisms governing hepatocyte replication during liver regeneration. The prolactin (PRL) receptor (PRLR) signaling system is highly conserved among all vertebrates, including ligand and receptor structure and downstream signaling molecules. PRLR signaling contributes to the regulation of numerous functions, especially those associated with reproduction and lactation. The liver is among several tissues with the highest PRLR expression. However, functions of hepatic PRLR signaling system are poorly understood. Our work and that of others have linked the PRLR signaling system to the regulation of hepatocyte expansion. Based on our preliminary studies, we centrally hypothesize that the PRLR signaling system exerts mitogenic signaling via yes-associated protein (YAP), stimulating hepatocyte proliferation during liver regeneration. We will test our central hypothesis and, thereby, accomplish the objective of this application by pursuing the following two specific aims. Specific Aim 1: Define the PRLR/YAP mitogenic signaling pathway in hepatocytes. Specific Aim 2: Determine the mode of action of the PRLR signaling system in regulating hepatocyte cell cycle during liver regeneration. The proposed studies will enable us to reveal how the PRLR signaling system acts as a novel and critical modulator that when activated stimulates hepatocyte expansion to compensate for lost liver mass in an injured liver. In addition, the proposed studies will provide essential information for evaluating the PRLR system as a new target for treating liver injuries through enhancing hepatocyte proliferation and thereby liver repair. 1